1. Technical Field of the Invention
This invention relates to masts and methods for raising or erecting masts, particularly on offshore drilling platforms or at relatively inaccessible sites on land.
2. Description of the Prior Art
Several types of well masts have been used in connection with oil wells. Often, these masts have been portable for movement between well sites, particularly for offshore or remote area drilling. It has been customary to have masts of telescoping sections which can be extended one above the other to make a mast of desired height.
One type of mast took the form of two section telescoping masts with an upper section telescoped into a lower section. The entire assembly was transported as a unit from one location to the next. The telescoped mast sections as a unit made the unit heavy. Further, on arrival at the drilling site, the entire collapsed assembly was tilted up as a unit into vertical position, at which time the upper section was then extended. Tilting of the entire mast as a unit was undesirable both for load reasons and in that it caused damage and increased wear and tear on the mast structure.
A second type of portable mast used a three-section telescoping mast with an intermediate section and an upper section telescoped within a lower section. Other than the added section, the three-section mast operated in the manner of the two-section masts. Because of the added section, weight, load and hauling problems only increased.
U.S. Pat. No. 4,134,237, of which applicant Armstrong is an inventor, issued Jan. 16, 1979 and disclosed a third type of portable mast. This mast comprised a modular section mast having a lower or base section first mounted in a vertical position on a substructure. Upper and, if required, intermediate sections, were positioned one at a time as additional sections within the open side of the fixed vertical lower section. The additional sections were then extended upwardly, one at a time, from the lower section to form a fully extended mast of desired height. This type of mast, although affording advantages over telescoping masts, had several undesirable features. At the drilling site, the sections were stored in a horizontal position. Erection of the mast required lifting of each of the modular sections from the horizontal position to a vertical upright position prior to installing them in the lower section.
This latter type of mast structure suffered problems when raising the mast sections from the horizontal storage position to the upright position. In raising the mast sections, they were connected at the end to be the upper end with a lifting cable while in the storage position. As the lifting cable raised this end of the mast section, the other or free end of the lifted mast section dragged across the drilling platform or other well site surface, damaging both the well site surface and mast structure.